The production of methane gas by microbial fermentations is a well-established technology with many commercial applications in waste treatment and biomass conversion for over 100 years. By contrast, hydrogen (H2) fermentations—that is, the anaerobic conversion of organic substrates to H2 gas—have not been developed to practical applications despite a great deal of research (reviewed in Hallenbeck and Benemann, 2002; see Literature Cited) and many patents (see Patents Listed) covering various methods of H2 fuel production by fermentations. One major reason for this relative lack of success is that the yield of H2 has been rather low, generally less than 20% on an energy (fuel) content basis compared to the yield of methane or ethanol fermentations under similar conditions. That has discouraged practical applications.
An extensive literature exists on two-stage, also called “two-phase”, methane fermentations, which produce H2 gas in the first stage (or phase) and methane gas in the second stage. However, the prior art does not disclose the harvesting of both H2 and CH4 or of H2—CH4 mixtures in an integrated process. Indeed, in most of the descriptions in the literature, no mention is made of the nature of the gas produced in the first stage, which is often not even analyzed. Furthermore, often the gas from the first phase is transferred into the second, to allow any H2 produced in the first stage to be converted to methane gas in the second, which is the only fuel produced by such anaerobic digestion systems.
In certain applications the production of H2—CH4 mixtures could be of interest, in particular due to the reduction in NOx and other pollutants that result from the combustion of such mixtures in engines, compared to CH4 combustion alone. Thus combustion of such H2—CH4 mixtures is of interest in use of biogas for electricity generation or transportation applications wherever air pollution is an issue. Thus, it has been proposed that two-stage fermentations that produce both H2 and CH4 could be also used to produce such H2—CH4 mixtures for air pollution reduction (in particular NOx reduction) purposes (Benemann, 1996, 1998). However, for practical applications methods must be developed to allow for control over the relative volumes of these two gases such that the combined gas composition can be maintained at a desired level for the application at hand, even when the fermentation substrate (feed), changes in nature or flow, or both, or when other performance parameters in the reactor, such as acidity, temperature, gas flow, or composition, change, resulting in reduced or excessive H2 production. For many applications it would be desirable to produce H2 gas at or near the maximum amount that is feasible by such fermentation processes, with or without admixture of the CH4 gas produced in the second stage. This objective requires optimization of the operating conditions, in particular of the hydraulic retention time and loading rate as well as of the H2 concentration, and other performance parameters in the first stage of the process. A process for accomplishing these objectives is described herein.